Wollaston prism-based structured illumination microscope with tunable frequency

Management of the axial modulation of the illumination pattern in structured illumination microscopy using an extended illumination source

We have designed and implemented an approach for three-dimensional (3D) structured illumination (SI) microscopy (SIM) based on a quasi-monochromatic extended source illuminating a Wollaston prism to improve robustness, light efficiency and flexibility over our previous design. We show through analytical and experimental verification of the presented theoretical framework for our proposed tunable structured illumination microscopy (TSIM) system, that a simple and accurate determination of the axial modulation of the SI pattern is achieved, enabling a realistic characterization of the system's effective optical transfer function (OTF). System performance as a function of the extended source size is investigated with simulations. Results from a comparative performance analysis of the proposed TSIM system and traditional SIM systems show some advantages over the traditional two-wave and three-wave interference SIM systems. We show that by controlling the source size and thereby the axial modulation of the 3D SI pattern, the TSIM scheme offers increased OTF compact support and improved optical sectioning capability, quantified by the integrated intensity, under certain conditions, which may be desirable when imaging optically thick samples. The additional tunability of the 3D SI pattern, provides a unique opportunity for OTF engineering in our TSIM system.

Broadband Wollaston prism with a large output beam separation based on mercurous halides

The paper proposes a Wollaston-type crystal polarizer suitable for broadband operation within the visible spectral band up to the far infrared band based on unique optical materials, mercurous halides (Hg₂X₂). This paper introduces the general characteristics and optical properties of these birefringent tetragonal optical materials, as well as the general description of a Wollaston prism and the process of its parameter optimization. In general, the Wollaston polarizer is constructed from two combined wedge-shaped prisms. The key parameters that affect the properties of the Wollaston polarizer are then the cut angle of these two prisms and the refractive index of the exploited optical cement (immersion) that bonds the prisms together. The optimal prism cut angles and immersion refractive index are investigated to maximize the Wollaston parameters, such as the transmittance of the polarized radiation and the separation angle of the output orthogonally polarized beams. This process is significantly dependent on the characteristics of all selected mercurous halides (Hg₂Cl₂, Hg₂Br₂, Hg₂I₂). The optimal values of the prism cut angle for each material are selected based on the outlined results. In addition, the Wollaston prism behaviour regarding real radiation propagation is modelled in detail via the Zemax optical studio. The presented models aim to aid in the real design and fabrication of a broadband Wollaston polarizer based on mercurous halides.

Advantages of Fresnel biprism-based digital holographic microscopy in quantitative phase imaging

SIGNIFICANCE

The hallmarks of digital holographic microscopy (DHM) compared with other quantitative phase imaging (QPI) methods are high speed, accuracy, spatial resolution, temporal stability, and polarization-sensitivity (PS) capability. The above features make DHM suitable for real-time quantitative PS phase imaging in a broad number of biological applications aimed at understanding cell growth and dynamic changes occurring during physiological processes and/or in response to pharmaceutical agents.

AIM

The insertion of a Fresnel biprism (FB) in the image space of a light microscope potentially turns any commercial system into a DHM system enabling QPI with the five desired features in QPI simultaneously: high temporal sensitivity, high speed, high accuracy, high spatial resolution, and PS. To the best of our knowledge, this is the first FB-based DHM system providing these five features all together.

APPROACH

The performance of the proposed system was calibrated with a benchmark phase object. The PS capability has been verified by imaging human U87 glioblastoma cells.

RESULTS

The proposed FB-based DHM system provides accurate phase images with high spatial resolution. The temporal stability of our system is in the order of a few nanometers, enabling live-cell studies. Finally, the distinctive behavior of the cells at different polarization angles (e.g., PS capability) can be observed with our system.

CONCLUSIONS

We have presented a method to turn any commercial light microscope with monochromatic illumination into a PS QPI system. The proposed system provides accurate quantitative PS phase images in a new, simple, compact, and cost-effective format, thanks to the low cost (a few hundred dollars) involved in implementing this simple architecture, enabling the use of this QPI technique accessible to most laboratories with standard light microscopes.